That's the problem. If there was a well-known user named "foobar@lemmy.world" and that instance went down, that would be a perfect opportunity for some other random person to create an account named "foobar@sh.itjust.works" and claim to be their alt account that they are using while their main account is unusable. As we can see from this thread, genuine users definitely do use alts on different instances to deal with downtimes like this, so that is a plausible cover for an impersonator.